1. Field of the Invention
The present invention relates to methods and compositions for reducing the permeabilities of subterranean zones, and more particularly, to improved water soluble polymeric compositions which form cross-linked gels in the zones.
2. Description of the Prior Art
When wells penetrating oil and gas producing subterranean formations are produced, water often accompanies the oil and gas. The water can be the result of a water producing zone communicated with the oil and gas producing formation by fractures, high permeability streaks and the like, or it can be caused by a variety of other occurrences which are well known to those skilled in the art such as water coning, water cresting, bottom water, channeling at the well bore, etc.
In enhanced recovery techniques such as water flooding, an aqueous flood or displacement fluid is injected under pressure into an oil containing subterranean formation by way of one or more injection wells. The flow of the aqueous fluid through the formation displaces oil contained therein and drives it to one or more producing wells. However, the aqueous displacement fluid often flows through the most permeable zones in the subterranean formation whereby less permeable zones containing oil are bypassed. This uneven flow of the aqueous displacement fluid through the formation reduces the overall yield of hydrocarbons from the formation.
Heretofore, enhanced recovery problems in a subterranean oil containing formation caused by permeability variations therein have been corrected by reducing the permeability of the subterranean formation flow paths having high permeability and low oil content. As a result, the subsequently injected aqueous displacement fluid is forced through flow paths having low permeability and high oil content. The techniques utilized to accomplish this high flow path permeability reduction, referred to in the art as "conformance control techniques," have included injecting aqueous solutions of polymers and gelling agents into the high permeability flow paths whereby the polymers are gelled and cross-linked therein. For example, water soluble polymers including copolymers of acrylamide and acrylic acid cross-linked with chromium or other transition metal ions have been utilized heretofore. In accordance with an early technique, an aqueous solution of one or more of the polymers or copolymers mixed with a cross-linking metal ion is injected into the subterranean formation and allowed to cross-link therein. However, it has heretofore been found that the cross-linked gels formed have often been ineffective at high temperatures, i.e., at temperatures above about 80.degree. C. because of the instability of the cross-linker or polymer. This has resulted in uncontrolled cross-linking rates (too rapid), cross-linker precipitation, polymer degradation, or an inefficient solution propagation. In attempts to correct these problems, the cross-linking metal ion has been coordinated with a ligand such as acetate or propionate to slow the reaction of the metal ion with the polymer. While this and other techniques have been utilized successfully, the use of some metal ions, e.g., chromium, has adverse environmental effects, and the metal ion used can be adsorbed by formation materials whereby it is prevented from functioning to cross-link the polymer.
U.S. Pat. No. 4,773,481 to Allison et al. issued on Sep. 27, 1988 describes a process for reducing the permeability of a subterranean formation by the cross-linking of water soluble polymers of polyalkylene imines and polyalkylenepolyamines with certain polymers which are anionic or hydrolyzable to form anionic polymers. Examples of the anionic polymers are polyacrylamide and alkylpolyacrylamides, copolymers of polyacrylamide and alkylpolyacrylamides with ethylene, propylene and styrene, polymaleic anhydride and polymethylacrylate and hydrolysis products thereof. As described in the patent, when the water-soluble polymer and the anionic polymer are mixed, a viscous gel is quickly formed. In use, a solution of the water-soluble polymer is pumped into the subterranean formation first, followed by water to displace the water soluble polymer from the well bore to thereby prevent premature gelling upon introduction of the anionic polymer. Thereafter, the anionic polymer is pumped into the formation. This three step procedure has a number of disadvantages in practice and is costly to perform, but it is necessary because the water soluble polyalkylene imine or polyalkylenepolyamine reacts very quickly with the anionic polymer and cannot be premixed without premature gelation.
Thus, there are continuing needs for improved methods and compositions for reducing the permeabilities of subterranean zones using water soluble polymeric components whereby the cross-linking of the components is effectively and simply controlled at high temperatures.